Due to the growing demand for energy efficiency, semiconductor devices such as phase control thyristors are at the heart of much of the equipment needed for energy transmission and distribution. These devices allow good performance in terms of cost, reliability and efficiency. Exemplary, bipolar semiconductor power devices are used in applications for their very high power capabilities combined with very low conduction losses. For instance, for a non-punch through device type, a low-(n−) doped drift layer, i.e. an (n−)-base region, which is the thickest layer of the device, cannot be reduced below a certain limit. However, the thickness of a p doped first layer, i.e. a p-anode or a p-base region, can be reduced. This can be beneficial especially in the case of a negative bevel junction termination, which for several reasons consumes a big volume in a transversal direction and the first layer must be therefore thicker than in a positive bevel concept.
The junction termination with negative bevel is of practical importance, because contrary to the positive bevel, it keeps peak electric fields at blocking conditions inside the device. As a result, the surface passivation is not exposed to extremely high electric fields, the surface leakage current is smaller and a high reliability is achieved. This is of great importance as for instance at the conditions of avalanche lightning in a High Voltage Direct Current HVDC application high magnitudes of reverse current are generated at the avalanche reverse breakdown. In other words, maintaining the negative bevel has the advantage of robust reverse blocking up to the breakdown voltage, when significant current may flow through the junction termination at the periphery, but still not approaching the surface of a device. Therefore, the use of a negative bevel is needed for such applications. However, thyristors need to be developed with focus on minimizing overall losses and maximizing power rating of the device.
In order to maintain both the forward and reverse blocking at the level of a prior art device with thick anode and base layer, local deep p doped termination layers can be used at the termination region. The deep termination layers allow to have junction terminations with single or double negative bevel, which in principle provides high avalanche lightning capability required in the HVDC applications.
Earlier thyristor designs 10 for instance according to WO 2012/041836 A1 suggested a wafer 2 comprising an inner region 7 and an outer region 8. On a first main side 3, a p doped first layer 6 having a first section in the inner region 7 and a second section 62 in the outer region 8 is arranged on an (n−) drift layer 5. The second section 62 of the first layer 6 has according to the prior art a depth much greater than the first section 61 of the first layer 6. Both sections 61, 62 extend on the first main side 3 up to the same planar plane, only declines in the outer region 8 a the negative bevel angle. On a second main side 4 opposite to the first main side 3, a second layer 16 is constructed in the same way as the first layer 6 with a first section 161 and a second section 162. On one of the main sides 3, 4, p+ doped shorts 18 and n+ doped cathode layers 23 are arranged in the p doped first section 61 or 161, these layers 18, 23 contacting the electrode 35 or 45. In the outer region 8, a negative bevel terminates the device towards the edge of the wafer 2.
Such a design of the first and second layers 6, 16 leads to a reduced thickness of the first sections 61, 161 in the inner region compared to other prior art devices without increasing the leakage current and decreasing the breakdown voltage, and brings lower on-state voltage drop VT. Also other parameters like reverse recovery charge Qrr, turn-off time tq and maximal surge current are improved. The total device thickness can be reduced due to the thinner first section of the first and second layer 6, 16, while the reverse and forward blocking capability is maintained by means of the modified junction termination with lightly-doped P-type termination layers and negative bevel.
However, the earlier design leads to locally high electric fields within the device as the thickness of the first layer 6 changes from a first thickness in the inner region 7 to a second thickness in the outer region 8, which may be several times as thick as the first thickness. During the production process a step variation of the first layer 6 from the inner region 7 to the outer region 8 is created on the junction to the drift layer 5. This variation in a boundary between the (n−) doped drift layer 5 and the p doped first layer 6 causes locally larger electric fields and higher leakage currents of the device.
EP 0 30 046 A1 describes a GTO thyristor, which comprises a thin, but highly doped central p-layer and a thick, but lowly doped outer p layer, which projects the central p layer. In the outer region, the thyristor has a negative bevel.